1. Field of the Invention
The present invention relates to digital to analog converters (DACs), and in particular relates to DACs with self-calibration functions.
2. Description of the Related Art
FIG. 1 depicts a conventional DAC 100, which converts a digital code Din to an analog value (output voltage Vout). As shown in the figure, a voltage Vref is forced to a resistor R to determine a current Ipd. The current Ipd is provided by a transistor M1, wherein the gate voltage Vcon is coupled to the gates of the transistors M21-M2n to drive the transistors M21-M2n to generate currents. According to the digital code Din, a switching circuit 102 selectively couples the transistors M21-M2n to a current to voltage converter (realized by a resistor Rout). The output current Iout is dependent on the digital code Din. Because the value of the output voltage Vout is a product of the output current Iout and the resistor Rout and the output current Iout is dependent on the digital code Din, the output voltage Vout reacts to the digital code Din.
However, process variations during the IC manufacturing process, such as the channel length/width mismatch, mobility mismatch or threshold voltage mismatch, may affect the accuracy of digital to analog conversion.
U.S. Pat. No. 6,157,332 discloses DACs with self-calibration functions, as shown in FIG. 2. During the self-calibration process, the digital code Din is of a specific value, the control logic 202 generates a self-calibration signal 204 to turn on the switch 206 and to turn off the switch 208. The amplitude detector 210 detects an output voltage Vout from an n-bit DAC 212 to generate an UP signal 214 and a DOWN signal 216 to control the charge pump 218 to charge or discharge the capacitor 220, so that the voltage Vref is adjusted and the output voltage Vout is affected.
For the DAC of the U.S. Pat. No. 6,157,332, however, current leaks out of the capacitor 220. Thus, the self-calibration procedure has to be periodically repeated to maintain the accuracy of the voltage value Vref. The demand for periodic self-calibrations, limits applicability of the DACs in electronic devices with periodic synchronization periods, such as a video display device. Furthermore, circuitry for the charge pump 218 and the selective output signal coupler 222 of the DAC is complicated, leading to relatively higher costs.